The initiation and maintenance of the differentiated state requires selective transcription of DNA. Major problems in molecular and developmental biology include the mediation and regulation of the vast changes in gene transcription occurring during cytodifferentiation. Chick skeletal muscle myogenesis is chosen as a model system in which to approach these problems. This proposal delineates methods of analyzing the structure and activity of a single gene -- the skeletal muscle myosin heavy chain (MHC) gene -- during development. Methods are described to complete the analysis of purified MHC messenger RNA, and to use this RNA as a template to synthesize a complementary DNA (MHC-cDNA). MHC cDNA-H3 will be used, via DNA-RNA hybridization, as a specific probe for detection of MHC messenger RNA in myogenic cells as they approach and proceed through their final developmental transition. Although chromatin has been fractionated into putative "active" and "inactive" components, possible changes in gene structure (shifts from "inactive" to "active" chromatin) during development have not been documented. I will use MHC-cDNA as a probe to detect the MHC gene in chromatin fractions isolated from cells at selected stages in myogenesis. Finally, the thymidine analog 5-BrdU is a specific inhibitor of myogenesis. As such it should be a valuable probe to perturb myogenesis, eventually leading to a better understanding of the normal process. The methods described above are uniquely suited to an analysis of the mechanism of BrdU inhibition.